bad 
- tions of declination are attributable to the annual variations in 
be 
ing that the law of the variation is the same whatever place so 
“ sitnated with regard to Philadelphia be taken. The following » 
_ than that at Philadelphia at the beginning He: yea 
oa to it, or greater, toward the middle of ‘the year, sed less 
have moved towards the north during the first part of the year, 
4 #% 
338 Prof. W. A. Norton on the Diurnal and Annual 
drawn from them; I anni accordingly reserve this discussion for 
a future occasion. © 
Annial Variations of Declination. 
Ascordive to the theory under consideration the annual varia- 
the position of the isothermal line—or ‘ra ther, to be ‘precise, of 
en line of equal molecular magnetic intensity, Bs must ap- 
ximate more or less to the isotherm al line. w the annual 
peavermies of the isothermal line may be nated by diseuss- 
ing the annual LY set ‘that take place in the differences be- 
tween the» mean monthly temperature at Philadelphia and at 
some place to the. baat or west of Philadelphia ;—that is, suppos- _ 
table’ shows the méan monthly differences of temperature for 
1841, = de Piialclphia and several dese to the west of 
Philad elphi 
son ‘Melis Differences of: Péiiérature for 1841. 
———— 
oo ie ae Jan, Feb. ee April.| May. |June. | July.|A’g.! Sept — ee ~- 
Phila. and Lancaster, _../3°38/1°59| 0-06 | 0-23| 0°38| 0911...) 0-931 1-501 41 
~*~ and Gettysb’g,|....|5-23/3-44| os 0-09] 1°89} 2:18..,. 1 eihae 5-0914°70) 
“— aiid Gaile, mee ry 19 |-15} so eeeh. . «pp 280.159 
“and Harrisb’g,|... 19 ba2l-ovo vee [28 ELT boa F 45 | a [47 
sy: and .Chest’r oo 22 13 + lace sent ee eleeeer nceneieese 
will be. observ sabe 5 5 the mean monthly differences. of. tem- 
cates for all these places are greater at the beginning and end 
of the year than: toward the m iddle ¢ the yea r. .The minus 
sign a a the temperate at iladel pia is less. than at 
the other p § um 
It a ots these results that during the yet aft, the 
temperature at places to the west of Philade ng I 
at the ‘close of the year. The isothermal line through 
and towards the south during the latter part of the year. The > 
tendency of these movements would be to make the declination . 
least toward the middle of the year and greatest at the beginning — 
and end of the year. Now, as a matter of fact, on axe 
the curve given in the Report of the Observations at the 
College Observatory, showing the annual variations of deatitms 
tion at Philadelphia, we find that in 1841 the declination was 
—— in January and February, and again in November 
I ee ay. On inspecting the 
